Handle null values in joined summary columns

src/BenchmarkDotNet/Reports/SummaryTable.cs

@@ -61,7 +61,7 @@ internal SummaryTable(Summary summary, SummaryStyle? style = null)
             ColumnCount = columns.Length;
             FullHeader = columns.Select(c => c.GetColumnTitle(style)).ToArray();
 
-            FullContent = summary.Reports.Select(r => columns.Select(c => c.GetValue(summary, r.BenchmarkCase, style)).ToArray()).ToArray();
+            FullContent = summary.Reports.Select(r => columns.Select(c => c.GetValue(summary, r.BenchmarkCase, style) ?? string.Empty).ToArray()).ToArray();
             IsDefault = columns.Select(c => summary.Reports.All(r => c.IsDefault(summary, r.BenchmarkCase))).ToArray();
 
             var highlightGroupKeys = summary.BenchmarksCases.Select(b => b.Config.Orderer.GetHighlightGroupKey(b)).ToArray();
@@ -141,4 +141,4 @@ public enum TextJustification
             }
         }
     }
-}
+}

tests/BenchmarkDotNet.Tests/Reports/SummaryTableTests.cs

@@ -87,6 +87,17 @@ public void TextColumnWithRightJustification()
             Assert.Equal(SummaryTable.SummaryTableColumn.TextJustification.Right, table.Columns.First(c => c.Header == "Param").Justify);
         }
 
+        [Fact] // Issue #2656
+        public void JoinedSummaryHandlesMissingColumnValues()
+        {
+            var taggedSummary = MockFactory.CreateSummary(typeof(TaggedBenchmark));
+            var otherSummary = MockFactory.CreateSummary(typeof(OtherBenchmark));
+
+            var joinedSummary = Summary.Join([taggedSummary, otherSummary], default);
+
+            Assert.Equal(["", "tagged"], joinedSummary.Table.Columns.Single(c => c.Header == "Tag").Content);
+        }
+
         [Fact] // Issue #1070
         public void CustomOrdererIsSupported()
         {
@@ -476,6 +487,27 @@ public void DisplayThreadingDiagnoserConfigs_WhenDescriptorValuesAreZero()
             Assert.Equal(["-", "-"], lockContentionCount!);
             Assert.Equal(["-", "-"], completedWorkItemCount!);
         }
+
+        [Config(typeof(TagColumnConfig))]
+        public class TaggedBenchmark
+        {
+            [Benchmark]
+            public void Tagged() { }
+        }
+
+        public class OtherBenchmark
+        {
+            [Benchmark]
+            public void Other() { }
+        }
+
+        public class TagColumnConfig : ManualConfig
+        {
+            public TagColumnConfig()
+            {
+                AddColumn(new TagColumn("Tag", methodName => methodName == nameof(TaggedBenchmark.Tagged) ? "tagged" : null!));
+            }
+        }
         #endregion
     }
-}
+}